The present invention involves surfactant flooding for the recovery of hydrocarbon oils in which synthetic polyisobutylene sulfonates are employed in an alkaline aqueous media in conjunction with at least one other surfactant or co-surfactants, preferably a sulfonate, an alcohols or a nonionic surfactant, or some mixture thereof, to lower the interfacial tension between the surfactant solution and the hydrocarbon oil, i.e. crude oil, that is to be recovered from a subterranean reservoir. The aqueous surfactant solutions thus allow for enhanced recovery of crude oil from subterranean reservoirs.
Crude oil, i.e. hydrocarbons, which accumulate in and are produced from subterranean reservoirs, are recovered or produced through one or more wells drilled into the reservoir. Prior to producing the crude oil, the formation, a porous media, is saturated with crude oil and all the pores are filled with crude oil. The initial recovery of the hydrocarbons is generally accomplished by xe2x80x9cprimary recoveryxe2x80x9d techniques wherein only the natural forces present in the reservoir are utilized to produce the oil. In this primary recovery, only a portion of the crude oil is driven out of the pores by formation pressure. For instance, a usual condition upon depletion of the natural forces and the termination of primary recovery is that a rather large portion of the crude oil, typically more than half its original volume, remains trapped within the reservoir. Moreover, many reservoirs lack sufficient natural forces even to produce the oil by primary methods.
This phenomenon has long been known in the petroleum industry, and consequently, recognition of such a fact has led to the development and use of many enhanced oil recovery techniques. Most of these techniques involve injection of at least one fluid or gas into the subterranean reservoir to produce an additional amount of crude oil. These liquids include water, steam, a miscible gas such as CO2 or natural gas, or an immiscible gas such as nitrogen.
While other fluids can provide higher oil recovery, water is the most widely used and economical fluid of choice. Water flooding involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace the oil therein to a production system composed of one or more wells through which the oil is recovered. Nevertheless water does not displace oil with high efficiency because of the immiscibility of water and oil and because of the high interfacial tension between them.
It has long been recognized that this high interfacial tension existing between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which can negatively influence the amount of oil recovered by water flooding.
There are two principal mechanisms of enhancing the oil recovery of an injected fluid. These methods are increasing volumetric sweep efficiency of the injected fluid and increasing the oil displacement efficiency by the injected fluid. Both techniques involve the addition of chemicals which modify the properties of the injected fluid.
A very usefull technique for increasing the oil recovery of water has been to add surfactants to the flood water in order to effectively lower the oil/water interfacial tension and/or alter the wettability characteristics of the reservoir rock. This effective reduction in interfacial tension allows the deformation of crude oil droplets thereby improving the movement of the oil through the porous channels of the reservoir. Therefore, with the addition of the surfactants to the flood water, the interfacial tension is effectively reduced between the water and the reservoir oil, the oil droplets deform, coalesce and subsequently flow with the flood water toward the producing wells. It is generally accepted that the interfacial tension between the surfactant solution and the reservoir oil should be reduced to less than 0.1 dyne/cm for low-tension flooding to give effective recovery. In order to provide oil recovery that is effective and economically feasible, the goal is to reduce interfacial tension to 10xe2x88x923 dynes/cm.
Generally, these methods of surfactant flooding which employ the injection of flood water to which has been added one or more surface active agents or surfactants, into a reservoir and allowing the solution or emulsion of surfactants to sweep through the formation and displace or recover oil, are commonly referred to as surfactant water flooding or as low tension water flooding, the latter term having reference to the mechanism involving the reduction of the oil-water interfacial tension. This procedure may be followed by a polymer solution for mobility control and improved sweep efficiency.
Anionic surfactants are popularly utilized in such water flooding applications. For example, a paper by W. R. Foster entitled xe2x80x9cA Low-Tension Water flooding Processxe2x80x9d, Journal of Petroleum Technology, Vol. 25, February 1973, pp. 205-210, describes a technique involving the injection of an aqueous solution of petroleum sulfonates within designated equivalent weight ranges and under controlled conditions of salinity.
One problem encountered in water-flooding with anionic surfactants, and with petroleum sulfonates in particular, is that they tend to become depleted from the injected solution through precipitation as the solution moves through the reservoir. The surfactants tend to be lost as insoluble salts of ionic materials, such as polyvalent metal ions. This phenomenon, referred to as a lack of stability, is more often seen in so-called xe2x80x9chard waterxe2x80x9d or xe2x80x9chigh brinexe2x80x9d environments. High brine waters typically contain high concentrations of inorganic salts, generally over 2% NaCl, for instance, and over 0.5% CaCl2 and MgCl2 total. Indeed, some xe2x80x9chigh brinexe2x80x9d reservoirs may have concentrations of NaCl of over 4%, and concentrations of over 2% CaCl2 and MgCl2 combined. In particular, these surfactants tend to precipitate from solution in the presence of monovalent salts such as sodium chloride at relatively low concentrations in excess of about 2 to 3 weight percent, or in the presence of even lower concentrations of divalent metal ions such as calcium and magnesium ions. For example, divalent metal ion concentrations of about 50-100 ppm and above usually tend to cause precipitation of the petroleum sulfonates. Other depletion may be caused by the adsorption of the surface active agent on the rock surface of the reservoir, or by the physical entrapment of the petroleum sulfonates in the pore spaces of the rock matrix.
In any case, it is quite obvious that if the surface active agent is removed from the water flood solution as it moves through the reservoir, the agent is not available to decrease the interfacial tension at the oil/water interface, and quite naturally it would follow that the surfactant depletion reduces oil recovery efficiency.
In a surfactant flood oil recovery process where the water contains a surfactant, the efficiency of the oil recovery from the reservoir is strongly affected by (1) the rate of surfactant loss, or surfactant stability, and (2) the surface activity of the surfactant, or in other words, the extent to which the interfacial tension is lowered at the oil/water interface. The lower the interfacial tension, the more efficient the recovery.
The tendency of surfactantants, and in particular of petroleum sulfonates to be depleted from the injected solution, also referred to as a lack of stability, has long been recognized in the art as a problem. Many suggestions have been proposed to overcome such problems. For instance, U.S. Pat. No. 3,637,017, Gale et al. describes a process which employs sodium petroleum sulfonate surfactants having average molecular weights within the range of 465-480 and alcohols, including aliphatic alcohols from 1 to 8 carbon atoms. In the method described in Gale et al., an aqueous solution of a petroleum sulfonate is injected into the formation and the surfactant is then displaced with water containing minor quantities of alcohol. Minor quantities of alcohol may be found in the surfactant solution as well as in the displacing water.
U.S. Pat. No. 3,997,451, Plummer et al., describes a combination of two different petroleum sulfonates having an average equivalent weight within the range of about 390-450, and having an aliphatic to aromatic proton (A/AP) ratio within the range of 4-20 moles per mole but having a difference in their respective A/AP ratio of at least 2.5 moles per mole for recovering increased amounts of oil.
U.S. Pat. No. 4,042,030, Savins et al., describes water thickened by the addition of an alkyl aryl sulfonate surfactant having a molecular weight from 350-500 and a C4-C6 aliphatic alcohol having a hydrocarbon chain link of at least 3 carbon atoms for use in a water flooding oil recovery process. The method of oil displacement in Savins et al. is not a microemulsion flooding in which the microemulsion acts to displace oil by means of a miscible displacement process, and has different composition requirements than a microemulsion system. See col. 11, lines 25 to 37. Microemulsion systems utilize higher concentrations of surfactants and as such must address precipitation issues differently.
The use of certain combinations of anionic and nonionic surfactants to combat hard water formations has also been suggested. For example, U.S. Pat. No. 3,811,505 discloses the use of alkyl or alkylaryl sulfonates or phosphates and polyethoxylated alkyl phenols. U.S. Pat. No. 3,811,504 discloses the use of three component mixture including an alkyl or alkylaryl sulfonate, an alkyl polyethoxy sulfate and a polyethoxylated alkyl phenol. U.S. Pat. No. 3,811,507 discloses the use of a water soluble salt of a linear alkyl or alkylaryl sulfonate and a polyethoxylated alkyl sulfate.
U.S. Pat. No. 4,110,229, Carlin et al., describes an aqueous, saline surfactant-containing fluid and an oil recovery process using the fluid for recovering oil from formations containing high salinity and/or high hardness water. The fluid comprises an anionic surfactant such as organic sulfonate especially petroleum sulfonates as well as synthetic alkyl or alkylaryl sulfonate and a solubilizing co-surfactant such as an ethoxylated alcohol, alkyl phenol, or alkyl or alkylaryl thiol, or sulfated or sulfonated, ethoxylated alcohols or alkyl phenols. Carlin et al. describe that optimum performance is achieved if the ratio of primary anionic surfactant to solubilizing co-surfactant is carefully chosen to exhibit borderline solubility in the particular formation brine in which the surfactants will be employed.
The use of certain amphoteric surfactants which function as cationics in acid media and become anionic when incorporated in alkaline systems has been suggested. For example, U.S. Pat. No. 3,939,911 discloses a surfactant water flooding process employing a three-component surfactant system. This surfactant system includes an alkyl or alkylaryl sulfonate such as an ammonium dodecyl benzene sulfonate, a phosphate ester sulfonate, and a sulfonated betaine such as a C12-C24 alkylamido C1-C5 alkane dimethylanuonium propane sulfonate.
In addition to the stability issues raised with the use of petroleum sulfonates, the supply of these natural sulfonates is limited. Oil recovery processes utilize billions of pounds per year. It is recognized in the art that there remains a need for surfactants that effectively reduce interfacial tension between the flood water used in the oil recovery process and the crude oil, exhibit improved oil recovery and improved solution stability, and yet remain economical and available to use.
Surprisingly, the surfactant system of the present invention, comprising a at least one synthetic polyisobutylene sulfonate, and a secondary surfactant which preferably is a sulfonate, an alcohol, and/or a nonionic surfactant result in surfactant solutions that when used in oil recovery processes, produce ultra-low interfacial tension at the water/oil interface, and have excellent solution stability of greater than 24 hours without precipitation, so as not to deplete from the surfactant solution during the oil recovery process.
The present invention relates to an aqueous fluid useful for the recovery of a liquid hydrocarbon from subterranean reservoirs comprising an aqueous media, and a surfactant blend. The surfactant blend comprises at least one synthetic polyisobutylene sulfonate. The secondary surfactant is preferably a sulfonate, an alcohol, a nonionic surfactant, or some combination thereof. The aqueous fluid has an alkaline pH of greater than 7.
The surfactant system of the present invention, when added to a waterflood is capable of reducing interfacial tension between a liquid hydrocarbon, i.e. crude oil, and the now formed surfactant waterflood to an ultra-low value.
The interfacial tension between the alkali/surfactant waterflood and the hydrocarbon oil such as crude oil, may be about 0.001 dynes/cm at the water/oil interface or lower. The stability of the resultant surfactant solution is 24 hours or greater without precipitation.
The present invention further relates to a method of recovering oil from a subterranean reservoir penetrated by a well bore by providing an aqueous fluid into the reservoir and contacting oil with the aqueous fluid. The aqueous fluid comprises at least one polyisobutylene sulfonate surfactant and a secondary surfactant. The secondary surfactant is preferably a sulfonate, an alcohol, a nonionic surfactant, or a mixture thereof. The interfacial tension between the aqueous fluid and the oil is effectively lowered for efficient oil recovery. The aqueous fluid has an alkaline pH, and a polymeric thickener may be optionally added to the aqueous fluid for more efficient recovery.